1. Field of Invention
This invention generally relates to a transmissive, rear-illuminated liquid crystal display (LCD) having unique retardation films (compensating films) that increase the effective viewing volume of the display.
2. Description of Related Art
Liquid crystal displays (LCD) are replacing other display devices, such as CRTs, as LCD technology matures. Most liquid crystal (LC) materials are uniaxial. Uniaxial materials possess one unique axis, the optic axis, which is parallel to the long axis of the LC molecules. LC materials are also anisotropic, which gives them the optical property of birefringence. Birefringence is the phenomenon of light traveling with different velocities in crystalline material, depending on the propagation direction and orientation of polarized light relative to the crystalline axes. This implies an index of refraction, n.sub.e, for the unique or extraordinary direction that is different than the index of refraction n.sub.o for the ordinary direction. When .DELTA.n=n.sub.e -n.sub.o is positive, the LC material is said to have positive birefringence.
FIG. 1 shows a conventional transmissive direct-view twisted-nematic LCD 100 that includes a backlight source 105 and an optical stack 106 that includes a number of layered optical elements that modify the polarization state and the spectral composition of light originating from the backlight source 105. The elements include rear polarizer 110 with a direction of polarizer transmitting axis indicated by lines 112, retardation films 120, LC cell 130, a voltage source 140, and an front polarizer 150 that serves as a polarization state analyzer.
The LC cell 130 includes LC material 138 disposed between a rear substrate 131 and a front substrate 132. Between the front and the rear substrates 132 and 131, respectively, and LC material 138 are front electrodes 134 and rear electrodes 133. Next are a rear polyimide film 135 and a front polyimide film 136. The voltage source 140 is connected to the rear electrodes 133 and the front electrodes 134.
LC cells such as LC cell 130 are defined by both the type of LC material used in the cell and the way light propagates through the cell. One such LC cell configuration is the twisted-nematic (TN) cell. Assuming the LCD 100 is a TN cell, light from backlight source 105 is linearly polarized by the rear polarizer 110 and then the optic axis is rotated by the TN LC cell 130. The amount of rotation is determined by the birefringence and thickness of the LC cell 130 as well as the twist angle of the LC cell 130 and the wavelength of the light. For example, many TN-LCDs use a 90-degree rotation. The 90degree rotation may be established by rubbing the polyimide films 135 and 136 and then orienting the polyimide films 135 and 136 such that the rubbing directions differ by 90 degrees. The LC molecules adjacent to the polyimide films 135 and 136 tend to align in the rubbing directions. Polyimide films may also be manufactured to provide a pre-tilt angle .theta..sub.o for the LC molecules within the LC cell 130. The pre-tilt angle .theta..sub.o ensures that the LC molecules tilt in the desired direction when a control voltage is applied to the front and the rear electrodes 134 and 133 by the voltage source 140.
After rotation of the optic axis, the polarization state of light exiting the LC cell 130 is analyzed by the front polarizer or analyzer 150. Because the polarization axes of the rear polarizer 110 and the analyzer 150 are crossed, the LCD will appear white when no voltage is supplied from the voltage source 140 to the electrodes 133 and 134. This configuration is called normally-white (NW).
Although the LC material 138 is homogenous, it is convenient to consider the LC material 138 as including a number of layers as shown in FIG. 1. Such terminology is well known in the art. Thus, FIG. 1 shows five layers of LC molecules, or directors, L1-L5. The optic axis of each layer L1-L5 is aligned with the axis of the director as shown in FIG. 1.
As shown in FIG. 1, the LCD 100 employs retardation films or compensating layers 120. As will be described later, the compensating layers 120 improve the viewing angle between the display and the viewer.
The LCD 100 operates in a number of "states". In the voltage "OFF" state, the directors within the LC cell 130 are aligned as shown in FIG. 1, and light is effectively transmitted through the optical stack 106. When the voltage from voltage source 140 to the electrodes 133 and 134 increases, the directors begin to tilt, and the twisted structure straightens. FIG. 2 shows the LCD 100 in the voltage "FULL-ON" state. Maximum tilt is achieved at the center of the LC cell 130 (such as L3) while the directors adjacent to the rear substrate 131 and the front substrate 132 experience relatively little tilt.
With the control voltage (typically 3-6 volts) applied, the optic axis of the central portion of the LC cell 130 is predominately parallel to the electric field and the twisted structure disappears as shown in FIG. 2. The polarized direction of the light is no longer rotated, and light passing through the LC cell 130 intersects the analyzer in the cross position, where it is absorbed, causing the activated portion of the display to appear dark.
FIG. 3 plots electro-distortion curves for a specific LC material. The curves show tilt angle .theta. and twist angle .PHI. as a function of the ratio of applied voltage V and a threshold voltage V.sub.c. As shown in FIG. 3, for a given control voltage, the tilt angle .theta. increases to a maximum value near a central region of the cell and is at a minimum value near the boundaries of the LC cell adjacent to the substrates. Also as shown in FIG. 3, the maximum tilt angle .theta. increases as the control voltage V to the electrodes increases. FIG. 3 also shows the effect of applying voltage to the electrodes in that the twist angle changes more rapidly with cell thickness in the central region of the LC cell compared to the boundaries of the LC cell.
One drawback to using LCDs is that contrast ratio and other optical characteristics degrade as the viewing angle increases. Several techniques have been developed to improve viewing angle performance for LCDs. Some methods of improving viewing angle performance include internal modifications such as in-plane switching mode (Kondo, SID 96 Digest, 81 (1996)), optical compensated mode (Miyashita, C.-L. Kuo, M. Suzuki and T. Uchida, SID 95 Digest, 797 (1995)) and multi-domain TN configurations Yang, IDRC 91 Digest, 68 (1991); J. Chen, P. J. Bos, D. L. Johnson, J. R. Kelly, J. Crow, N. D. Kim, SID 96 Digest, 650 (1996)). External modifications include a collimated backlight with a diffusing screen in front of the optical stack (McFarland, S. Zimmerman, K. Beeson, J. Wilson, T. J. Credelle, K. Bingham, P. Ferm, J. T. Yardley, Asia Display' 95, 739 (1995)). However, most of these techniques achieve viewing angle improvement at the cost of either greater manufacturing complexity or degraded optical efficiency. Hence, better ways to improve viewing angle performance without such compromises are highly desirable. One external method that shows more promise with fewer drawbacks is the use of compensating films (or retardation films).
Compensating films are available from a number of manufacturers, such as Nitto Denko, Sanritz and Fuji Film LTD. A typical material is polycarbonate. FIG. 4 shows an example of a compensating layer such as compensating layer 120. The compensating layer in this example is a polycarbonate film of thickness d.sub.r that has been stretched in both the x and y-directions such that n.sub.x =n.sub.y &gt;n.sub.z. When polycarbonate is stretched, the chain molecules tend to line up and the material is more polarizable (has a higher index of refraction) along the stretched axes. As a result, light that is polarized perpendicular to the stretch direction will propagate with an ordinary velocity. Another type of material is polystyrene. In the case of polystyrene, the polarizability of the material is greater in a direction perpendicular to the stretch direction. Polystyrene is typical of a material that has a negative birefringence (i.e., n.sub.e &lt;n.sub.o). However, compensating films such as compensating layer 120 provide compensation primarily for the LC material near the middle of the LC cell. Because the LC molecules near the LC cell substrates do not orient in a direction parallel to the applied electric field, these portions of the LC cell are not well compensated.
Yet another type of compensating layer is shown in FIG. 5. In FIG. 5, the compensating layer 180 includes CTA (cellulose triacetate) substrate 182, alignment layer 184, and discotic compound layer 186. The optic axis in the CTA substrate 182 is chosen so that it is normal to plane of the LC cell substrate. The optic axis of the discotic compound layer 186 is chosen so that it tilts over the thickness of the discotic compound layer as shown in FIG. 5. Thus, as shown in FIG. 5, the optic axis of the discotic compound layer 186 may change from 4.degree. to 68.degree., for example. This change in optic axis is designed to mimic to some degree the tilt of the directors in the LC cell. The compensating layer with this tilt can improve the performance of the viewing angle of a TN LCD. Such a compensating layer includes, for example, Fuji film WV Film WideView A.TM., described in "Optical Performance of a Novel Compensation Film for Wide-Viewing-Angle-TN-LCDs," Mori, Hiroyuki, Yuji Itoh, Yosuhe Nishiura, Taku Nakamurs, Yukio Shinagawa, AM-LCD' 96/IDW' 96, 189, which is hereby incorporated by reference.
FIG. 6 shows viewing angle characteristics of an LCD without a compensating layer. FIG. 6 is an iso-contrast diagram showing horizontal and vertical viewing angles and lines of constant contrast ratio. FIG. 7 is an iso-contrast diagram for the same LCD as in FIG. 6, but with compensating films such as shown in FIG. 5 placed adjacent to the LC cell. As can be seen by comparing FIG. 6 and FIG. 7, the LCD with the compensating film of FIG. 5 has a much better viewing angle performance than that of an LCD without the compensating film. Compensating films such as shown in FIG. 5 attempt to provide compensation for the LC material near the boundaries and the center of the LC cell.